I am not a lawyer but I don't read that as you have to present your US passport to enter or leave. Only that you must have a valid US passport with you when you do so.

>(b) Citizens

>Except as otherwise provided by the President and subject to such limitations and exceptions as the President may authorize and prescribe, it shall be unlawful for any citizen of the United States to depart from or enter, or attempt to depart from or enter, the United States unless he bears a valid United States passport.